|[AMRadio] Dummy load ideas|
k2pg at worldnet.att.net
Tue May 2 20:38:03 EDT 2006
Kanthal/Globar in Niagara Falls, NY makes some excellent noninductive
resistors that can be used in a high-power aircooled dummy load. They are
not cheap, but they will allow you to build a good dummy load that can
handle a kilowatt or more of steady carrier without the mess of oil. Add a
squirrel cage blower and you can make a dummy load comparable to the ones
sold for commercial use. Shielding the load with perforated metal stock and
grounding the shielding will prevent incidental radiation of your signal.
You can see the Kanthal product line at www.globar.com .
The problem with the Heathkit "Cantenna" and similar paint can loads made by
DenTron and MFJ is that the oil tends to migrate, making a mess. If the oil
overheats, it constitutes a fire hazard.
Light bulbs may be fine for tubed transmitters operating on relatively low
frequencies, but they are unsuitable for solid state transmitters and the
upper part of the HF spectrum. The reason is that the resistance of the
filament changes as it heats up. At full brilliance, the filament may
present a resistance of 50 ohms or so. But a cold filament represents a dead
short! On the higher frequencies, you also have a lot of inductive reactance
to deal with. Also, a light bulb will radiate over a short distance. I
worked someone years ago by loading up a bulb with a small ham rig!
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